Just get a cover note for insurance and after the two weeks don’t pay it unless you need to make a claim.
My understanding is that you can’t get any remaining rego, firstly you’ll need to register in his name and pay stamp duty, but as they aren’t from Vic they can’t register it.
Let seller refund plates, then for movement just get a unregistered vehicle permit, roughly $55 1 week, $66 two weeks, 3, $77 and max of 4 weeks for $88.
I think WA is the odd state with these laws, so check, but other states say that permit is to be obtained from state that journey starts. So in your case pay $55 to move it around Vic for a week without reg and then one permit in WA
Easiest option is to buy unreg, get unreg permit online for a week, fly to Melb and drive home within a week while on a cover note.